An acoustic logging tool transmits an acoustic pulse from a sonde supported transmitter to one or more receivers on the sonde. The pulse that is transmitted includes a number of cycles of acoustic energy. The travel path is in the vicinity of the well borehole and generally parallel to the sonde. The transmitted pulse is several cycles of an appropriate frequency for transmission in earth formations in the vicinity of the well borehole. At the receiver, there being one or more, the transmitted pulse is observed, and one aspect of receiving the acoustic pulse is to detect the various zero crossings which occur. Generally, the wave form of the transmitted pulse is more or less preserved at the receiver albeit subject to some measure of attenuation. The received pulse needs to be recognized at a particular portion of onset of the transmitted pulse. The measured transit time thus depends on starting measuring at a particular point in the transmitted pulse ending with a corresponding and similar point in the received pulse.
There is the possibility of jitter in transmission time dependent on whether or not the received pulse is measured at the proper instant. This depends on whether or not the particular zero crossing of the received acoustic pulse is recognized by the receiver circuitry. It is important to locate and recognize zero crossings. Zero crossing refers to the AC signal in the pulse crossing some reference, typically a zero reference, and counting to that zero crossing as the necessary reference. Cycle skipping is a problem in acoustic well logging. Cycle skipping refers to identification of an incorrect zero crossing event in the received acoustic pulse. Assume for easy description that the transmitted acoustic pulse has precisely ten cycles. Assume further that the very first cycle is a fully developed wave form. This defines a very sharp reference point in the transmitted pulse. At the receiver, if that particular zero crossing is observed, then there will be no error arising from cycle skipping. If, however, there is some loss of the first cycle (as for instance a failure to recognize the first cycle) at the receiver, then cycle skipping will occur where the receiver identifies the later cycles and affiliated zero crossings. In view of the relative short spacing and the relative rapid transit time, one cycle error can be a significant difference in acoustic transmission time.
This problem is magnified significantly when using long spaced tools because there is inherent additional attenuation of the acoustic signal along the longer travel path. The received signal must be amplified significantly. Even when amplified, it may be difficult to pick out the particular zero crossing event which corresponds with the particular event in the transmitted pulse. Thus, cycle skipping occurs in short transmission paths but is more likely for longer transmission paths. The method and apparatus of the present disclosure are directed to a system whereby the received acoustic pulse is evaluated to determine whether or not cycle skipping has occurred, and to improve the data which is obtained. The present apparatus and procedure are useful with different types of acoustic measuring devices. There are some systems which use two transmitters and other systems which use only one. The present disclosure accommodates both types of acoustic measuring devices.
The performance of an acoustic logging system is in part reported by describing the independent interval transit time, a factor known as Delta T. For a given interval to be acoustically tested and evaluated, the present system contemplates obtaining more than one Delta T measurement for that interval. In other words, two or more independent measurements are made. When the two values of Delta T are obtained by means of independent measurements, if a comparison is obtained, some indication of quality data is thereby derived. It is possible therefore to evaluate the logged data even where there are extremely high velocity contrasts between adjacent beds. In other words, there are certain geological formations which may show extremely high contrast between adjacent data on a curve of acoustic measurements. Sometimes, such recorded events may be wrongly rejected as possible cycle skips. It is very difficult to distinguish errors in the Delta T measurement as a result of cycle skipping without resort to the present invention. Another important factor is to create a quality factor which will indicate improved confidence in the acoustic log over an interval of interest on the log. If the quality factor is high, then it is assured that the data is the highest quality data which can be obtained. Even in the worst case situation, the quality factor still indicates that in some degree the obtained acoustic log is useful. Moreover, the present invention enables a procedure of eliminating the erroneous zero crossing events. In other words, data of such erroneous crossing events can be located and identified. If it is identified, it is then known that the data is of less than desirable quality. The foregoing is accomplished by construction of an important factor which will be described as the Delta T Discriminant, hereinafter DTD. In the preferred embodiment of the present system, it is ideally carried out by means of a computer which is provided with a program as described in the flow chart found herein, all for the purpose of providing substantially real time data evaluation in the operation of an acoustic log system. As mentioned above, it is adapted for use with both one and two transmitter systems.